The present application relates generally to improvements in differential to single-ended signal transfer circuits, and more specifically to techniques for increasing the open loop gain, for improving the AC performance, and for reducing the power supply voltage requirements of operational amplifiers including such transfer circuits.
Two-stage bipolar operational amplifiers are known that include a differential input stage and a single-ended second stage. For example, a conventional two-stage bipolar operational amplifier (op amp) may include a differential input stage, a differential to single-ended signal transfer circuit, a single-ended second stage, and an output buffer. In the conventional two-stage bipolar op amp, the differential to single-ended transfer circuit may comprise a current mirror, the single-ended second stage may comprise a common emitter circuit, and the output buffer may comprise a voltage follower circuit. Such a two-stage bipolar op amp typically has two high impedance nodes—one at the output of the first stage, and another at the output of the second stage. The conductance associated with these high impedance nodes, which is related to the transconductance of the first and second stages, determines the gain of the respective stages. For example, the gain of each stage may range from about 300 to about 1000, and the open loop gain of the two-stage bipolar op amp may range from about 100,000 to about 1,000,000.
To increase the operating voltage range of the conventional two-stage bipolar op amp, the output buffer may be omitted, and the output of the op amp may be provided at the second stage output. In this way, a two-stage bipolar op amp may be configured having a wide operating voltage range that approaches a rail-to-rail voltage swing. In this configuration, however, the load resistance is applied directly to the second stage output, which can reduce the gain of the second stage to about 10–100 based on the quiescent current level and the load resistance.
One way of increasing the open loop gain of a two-stage bipolar rail-to-rail op amp is to reduce the conductance at the output of the first stage (i.e., the differential input stage) and/or at the input of the second stage. To this end, the differential to single-ended transfer circuit comprising the current mirror may be provided with tracking feedback. For example, FIG. 1a depicts a differential to single-ended transfer circuit 100 comprising a current mirror 102, which includes a diode-connected input transistor Q1 and an output transistor Q2 coupled to the transistor Q1. The transfer circuit 100 further includes a transistor Q3 configured to provide tracking feedback for the current mirror 102, and a current source I0 connected to the respective emitters of the transistors Q1–Q3. As shown in FIG. 1a, the transfer circuit 100 has an inverting input INn at a common base connection 101 of the transistors Q1–Q2, a non-inverting input INp at a base connection 103 of the transistor Q3, and an output Vout at the base connection 103.
FIG. 1b depicts a generalized implementation 100a of the transfer circuit 100, in which a feedback amplifier 104 replaces the transistors Q1–Q3. As shown in FIG. 1b, the inverting input INn of the transfer circuit 100a, which corresponds to the base connection 101 of the transistors Q1–Q2 (see FIG. 1a), is connected to the inverting input of the feedback amplifier 104. Further, the non-inverting input INp of the transfer circuit 100a, which corresponds to the base connection 103 of the transistor Q3 (see FIG. 1a), is connected to the non-inverting input of the feedback amplifier 104. Like the transfer circuit 100, the output of the transfer circuit 100a is provided at the base connection 103. The transfer circuit 100a includes two current sources I1 and I2, both of which are controlled by the feedback amplifier 104.
In general, providing a current mirror with tracking feedback, as depicted in FIGS. 1a–1b, causes the input voltage of the current mirror to be equal to its output voltage, which is controlled by a common feedback loop. Because the input and output voltages of the current mirror are equal to one another, the input and output currents of the current mirror (e.g., the currents flowing through the transistors Q1–Q2, respectively; see FIG. 1a) are equal for any given output voltage Vout, thereby making the differential input well balanced. As a result, the open loop gain of the two-stage bipolar op amp incorporating the transfer circuit 100 or 100a (see FIGS. 1a–1b) can be effectively infinite.
One drawback of the transfer circuits 100 and 100a employing current mirrors with tracking feedback is that the input conductance associated with the single-ended second stage can cause the differential input stage to become imbalanced, thereby degrading the open loop gain of the two-stage bipolar op amp. Current gain can be increased by increasing the number of buffers between the input and the output of the second stage. However, because this approach increases the number of sub-stages within the second stage, the AC performance and the stability of the op amp may be adversely affected.
Another way of increasing the open loop gain of the two-stage bipolar rail-to-rail op amp is to employ a base current cancellation technique. For example, FIG. 2 depicts a differential to single-ended transfer circuit 200 with base current cancellation. The transfer circuit 200 includes transistors Q1–Q2, controlled current sources I1, and I2, and a current source I3. As shown in FIG. 2, the transfer circuit 200 has an inverting input INn at the base of the transistor Q2, a non-inverting input INp at the base of the transistor Q1, and an output Vout at the emitter of the transistor Q1. For example, such a base current cancellation technique may be employed to decrease the input current level of the transfer circuit 200 to about 1/30 of its initial level, while increasing the input impedance of the transfer circuit 200 by a factor of about 30.
However, the base current cancellation technique of FIG. 2 also has drawbacks. For example, due to the different collector-emitter voltages of the transistors Q1–Q2 of the transfer circuit 200, the betas of the transistors Q1–Q2 may be different, thereby causing an error in the base current cancellation. Further, the transfer circuit 200 is normally not suited for use with low power supply voltages due to the additional voltage drop across the transistor Q2. Moreover, the inverting and non-inverting inputs INn and INp of the transfer circuit 200 may be imbalanced, thereby exacerbating the base current cancellation error. In addition, in the event a boosted stage configuration of the transfer circuit 200 is employed (see, e.g., the transfer circuit 300 of FIG. 3), the additional phase shift created by the transistor Q2 within the local feedback loop of the transfer circuit 300 may degrade the AC performance and adversely affect the stability of the op amp.
It would therefore be desirable to have a two-stage bipolar rail-to-rail op amp including a differential to single-ended transfer circuit that avoids the drawbacks of conventional op amp circuit configurations.